223 
Abstract .—We investigated the 
spawning dynamics of southern blue- 
fin tuna, Thunnus maccoyii, using ova- 
ries obtained from fish caught on the 
spawning ground in the northeast In- 
dian Ocean and their main feeding 
grounds in the Southern Ocean, be- 
tween October 1992 and June 1995. 
Only sexually mature southern bluefin 
tuna were taken on the spawning 
ground and were caught in every month 
except July, although relative abun- 
dance was low from May to August. 
Peaks in abundance occurred during 
October and February. Individuals do 
not spawn over the whole season, and 
there is a turnover of fish on the spawn- 
ing ground. The presence of oocytes in 
all stages of development and the ab- 
sence of a hiatus in the oocyte size-fre- 
quency distributions between unyolked 
and early yolked oocytes indicate that 
southern bluefin tuna have asynchro- 
nous oocyte development and indeter- 
minate annual fecundity. The presence 
of either migratory nucleus or hydrated 
oocytes and postovulatory follicles in 
the ovaries of many females indicates 
that southern bluefin tuna are capable 
of multiple spawning. On the basis of 
the proportion of females with postovu- 
latory follicles, it appears that females 
spawn on average every 1.1 days. The 
average spawning batch fecundity, es- 
timated from counts of hydrated oo- 
cytes, was 6.0 million oocytes or 57 oo- 
cytes per gram of body weight. 
Manuscript accepted 1 July 1997. 
Fishery Bulletin 96: 223-236 (1998). 
Reproductive dynamics of 
southern bluefin tuna, 
Thunnus maccoyii 
Jessica H. Farley 
Tim L.O. Davis 
CSIRO Division of Marine Research 
GPO Box 1538, Hobart, Tasmania 7001, Australia 
E-mail address (for Jessica Farley): Jessica.Farley@marine.csiro.au 
Southern bluefin tuna, Thunnus 
maccoyii, are large, migratory pe- 
lagic fish with a circumglobal dis- 
tribution between 30°S and 50°S. 
Stocks are considered to be overex- 
ploited, and parental stocks, in par- 
ticular, are considered depleted 
(Caton et al., 1990; Anonymous 1 ). 
Of all the tunas, bluefin tunas ( T. 
maccoyii and T. thunnus) are the 
best adapted to cold water (Carey, 
1973) and thus are able to occupy 
the rich feeding grounds of the tem- 
perate oceans. However, like other 
tunas, the early life history of south- 
ern bluefin tuna is restricted to 
warm waters, and the only known 
spawning area is in the tropical east 
Indian Ocean where surface water 
temperatures exceed 24°C during 
the spawning period (Yukinawa and 
Miyabe, 1984; Yukinawa, 1987) (Fig. 
1). Juveniles leave the spawning 
grounds within a few months of 
hatching and move south along the 
continental shelf of Western Aus- 
tralia. This movement is probably 
assisted by the Leeuwin Current 
(Shingu, 1967; Maxwell and Cress- 
well, 1981). Juveniles first appear 
in the warm waters of the Great 
Australian Bight as one-years-olds 
in summer and then disperse along 
the West Wind Drift in winter. 
Ichthyoplankton surveys have 
been used to define southern blue- 
fin tuna spawning grounds and sea- 
son of spawning. Southern bluefin 
tuna larvae have been caught 
within latitudes 7° and 20°S and 
longitudes 102° and 124°E from 
September to March (Ueyanagi, 
1969; Yonemori and Morita, 1978; 
Yukinawa and Miyabe, 1984; Nishi- 
kawa et al., 1985; Yukinawa and 
Koido, 1985; Yukinawa, 1987; Davis 
et al., 1990). However, the northerly 
limit of spawning is not clear be- 
cause there has been little sampling 
effort in Indonesian territorial wa- 
ters. Apart from a general under- 
standing of the location and timing 
of spawning and the associated mi- 
gration patterns, little detailed in- 
formation is available on the repro- 
ductive dynamics of southern blue- 
fin tuna. To understand spawning 
seasonality, spawning frequency, 
and batch fecundity with greater 
accuracy, we analyzed ovarian 
samples from fish captured on 
spawning grounds in the northeast 
Indian Ocean and feeding grounds 
in the Southern Ocean (in waters 
off Tasmania, New Zealand, and 
South Africa) 
Materials and methods 
Southern bluefin tuna ovaries were 
obtained from two sources: Indone- 
sian-based longlining vessels fishing 
on the northern part of the spawning 
1 Anonymous. 1994a. Report of the thir- 
teenth meeting of Australian, Japanese 
and New Zealand scientists on southern 
bluefin tuna. Report to management, 20- 
29 April 1994, Ministry of Agriculture and 
Fisheries, Wellington, New Zealand, 13 p. 
